Electronic – What’s an Air Gap Layer in a PCB


At work I've inherited a multilayer PCB design that I need to send out for quote and eventual fabrication. It contains two inner layers that are labeled "AIRGAP". What is the purpose of these air gap layers?

The board stackup is as follows:
 1. Top Silkscreen
 2. Top Soldermask
 3. Top Copper
 4. Ground Layer
 5. Ground Layer Airgap
 6. VCC Layer
 7. VCC Layer Airgap
 8. Bottom Copper
 9. Bottom Solder mask

The highest voltage on the board is about 40 volts, so I wouldn't think it's a high-voltage design.

Would this be considered a four-layer board, or more? Some of the board houses we've sent it to are confused as well.

Best Answer

As Peter Bennett said, the air gap layer is probably a Gerber containing areas to be milled out of the layers, possibly the top and bottom prepreg, leaving the core intact. Since there are only 4 copper layers, this would likely leave open cavities on the top and bottom with copper potentially exposed on the power/ground layers.

This could be used to recess components into the PCB.

In some cases, components are completely embedded into the PCB.

I believe this process typically would have the (in this case) core run through a pick and place machine, soldered, cleaned and then laminated and the holes plated through with the top and bottom prepreg.

Here is an example of a stackup with completely embedded components from Altium:

enter image description here